System and method for receiving magnetic resonance (MR) signals with an FET electrically between preamplifier terminals

Abstract

A preamplifier is provided for a radio frequency (RF) receiver coil in a magnetic resonance imaging (MRI) system. The preamplifier includes an amplifier configured to receive at least one magnetic resonance (MR) signal from the RF receiver coil and configured to generate an amplified MR signal. An input circuit is electrically connected to the amplifier. The input circuit is configured to be electrically connected to an output of the RF receiver coil for transmitting the at least one MR signal from the RF receiver coil to the amplifier. The input circuit includes an impedance transformer and a field effect transistor (FET). The FET is electrically connected between the impedance transformer and the amplifier. The FET has an FET impedance. The impedance transformer is configured to transform a source impedance of at least approximately 100 ohms. The impedance transformer is further configured to transform the FET impedance into a preamplifier input impedance of less than approximately 5 ohms.

What is claimed is: 
     
       1. A preamplifier configured for a radio frequency (RF) receiver coil in a magnetic resonance imaging (MRI) system, the preamplifier comprising:
 an amplifier configured to receive at least one magnetic resonance (MR) signal from the RF receiver coil and configured to generate an amplified MR signal; and 
 an input circuit electrically connected to the amplifier, the input circuit being configured to be electrically connected to an output of the RF receiver coil and configured for transmitting the at least one MR signal from the RF receiver coil to the amplifier, the input circuit comprising;
 an impedance transformer; and 
 a field effect transistor (FET), with the FET being electrically connected between terminals of the preamplifier, the FET also being electrically connected between the impedance transformer and the amplifier, with the FET having an FET impedance, and the impedance transformer being configured to transform a source impedance of at least approximately 100 ohms, 
 the impedance transformer also being further configured to transform the FET impedance into a preamplifier input impedance of less than approximately 5 ohms. 
 
 
     
     
       2. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform the source impedance into an impedance that is within a noise circle of the FET. 
     
     
       3. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform a source impedance of at least 300 ohms. 
     
     
       4. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform the FET impedance into a preamplifier input impedance of between approximately 1 ohm and approximately 3 ohms. 
     
     
       5. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform an FET impedance of at least 500,000 ohms into the preamplifier input impedance. 
     
     
       6. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform the source impedance into an impedance that is within a noise circle of the FET, the noise circle of the FET having a size of at least 0.3 decibels. 
     
     
       7. The preamplifier of  claim 1 , wherein the impedance transformer is configured to transform the FET impedance into a preamplifier input impedance of approximately 2 ohms. 
     
     
       8. The preamplifier of  claim 1 , wherein the FET has an optimum reflection coefficient of less than 100 ohms. 
     
     
       9. The preamplifier of  claim 1 , wherein the impedance transformer is electrically connected between the RF receiver coil and the amplifier and is also configured to transform a coil impedance of the RF receiver coil into a source impedance of the amplifier. 
     
     
       10. A system configured for receiving magnetic resonance (MR) signals emitted by a subject, the system comprising:
 a radio frequency (RF) receiver coil configured to detect the MR signals; 
 a preamplifier configured to generate an amplified MR signal; 
 an impedance transformer electrically connected between the RF receiver coil and the preamplifier, wherein the impedance transformer is configured to transform a coil impedance of the RF receiver coil into a source impedance of at least approximately 100 ohms; and 
 a field effect transistor (FET) electrically connected between terminals of the preamplifier, with the FET also being electrically connected between the impedance transformer and the preamplifier, and the FET having an FET impedance. 
 
     
     
       11. The system of  claim 10 , wherein the impedance transformer comprises a lattice-type balun. 
     
     
       12. The system of  claim 10 , wherein the impedance transformer is configured to transform the coil impedance to a source impedance of at least 300 ohms. 
     
     
       13. The system of  claim 10 , wherein the impedance transformer is connected in series with the RF receiver coil. 
     
     
       14. The system of  claim 10 , wherein the impedance transformer is configured to transform a coil impedance of between approximately 2 ohms and approximately 10 ohms into the source impedance. 
     
     
       15. A system configured for receiving magnetic resonance (MR) signals emitted by a subject, the system comprising:
 a radio frequency (RF) receiver coil configured to detect the MR signals; 
 a preamplifier configured to generate an amplified MR signal; 
 an impedance transformer electrically connected between the RF receiver coil and the preamplifier, wherein the impedance transformer is configured to transform a coil impedance of the RF receiver coil into a source impedance of at least 100 ohms, wherein the impedance transformer comprises first and second inductors and first and second capacitors, the first inductor being connected to the first capacitor in series, the second inductor being connected to the second capacitor in series, the first inductor and first capacitor also being connected to the second inductor and the second capacitor in parallel; and 
 a field effect transistor (FET) electrically connected between terminals of the preamplifier, with the FET also being electrically connected between the impedance transformer and the preamplifier, and the FET having an FET impedance. 
 
     
     
       16. A method for receiving magnetic resonance (MR) signals emitted by a subject, the method comprising:
 receiving at least one MR signal from a radio frequency (RF) receiver coil at an impedance transformer; 
 transforming a coil impedance of the RF receiver coil to a source impedance of at least 100 ohms using the impedance transformer; 
 electrically connecting a field effect transistor (FET) between terminals of the preamplifier and electrically connecting the FET between the impedance transformer and a preamplifier, the FET having an FET impedance; and 
 amplifying the at least one MR signal using the pre-amplifier that is electrically connected to the impedance transformer. 
 
     
     
       17. The method of  claim 16 , wherein transforming a coil impedance of the RF receiver coil comprises transforming the coil impedance into a source impedance of at least 300 ohms. 
     
     
       18. The method of  claim 16 , wherein transforming a coil impedance of the RF receiver coil comprises transforming the coil impedance using a lattice-type balun. 
     
     
       19. The method of  claim 16 , further comprising transforming the source impedance, using the pre-amplifier, into an impedance that is within a noise circle of the FET the impedance having a size of at least 0.3 decibels. 
     
     
       20. The method of  claim 16 , further comprising transforming an FET impedance of the FET a preamplifier input impedance of less than 5 ohms using the pre-amplifier.